Military Tries Out Fish as Underwater Spies

They are prevalent, and nobody would suspect that they're being spied on by the fish.

Thought They Tried This several Times And Has Never Worked

That's an interesting thought, don't know how it would work out in reality and to control where that fish may be. Dolphins on the other hand can actually be trained and are thinking mammals although htye have to come up to breath unlike a fish. I'd have to know more about these programs, maybe But You would have to show Me a lot more to convince its worth the effort and expense.